Eulerian multifluid simulations of segregation and mixing of binary gas-solids flow of particles with different densities

Abstract

Dynamic characteristics and bubbling behavior of fluidized beds are known to influence the heat-/mass-transfer and reaction mechanisms in fluidized beds. In the present work, we have numerically investigated the effect of the local bubbling behavior on the dynamics of segregation and mixing of binary gas-solids flows with particles differing in density in a pseudo-2D fluidized bed and validated the predictions using time- and space-resolved measurements. We quantified the role of local bubbling behavior on the dynamics of segregation and mixing of binary gas-solids flows for varying bed compositions and superficial gas velocities. The closure model for solids frictional pressure and specularity coefficient in the wall boundary condition, that predicted bubbling characteristics correctly (smaller bubble size and bubbling frequency), led to satisfactory predictions of the segregation dynamics and agreed well with the corresponding measurements. For simulations of binary gas-solids flows, we show that the particle–particle drag model without the inclusion of momentum exchange due to particle velocity fluctuations under-predicted the time-evolution of mixing index. The model with long term frictional contacts led to a decrease in the segregation due to the increase in the magnitude of the particle–particle drag force and thus over-predicted the mixing index. Whereas, the modified Chao et al. (2012) drag model proposed in the present work, which includes the momentum exchange due to the particle velocity fluctuations and short-term particle frictional contacts, predicted the mixing index very well in comparison to the corresponding measurements.